The carbon nanotube (CNT) is a substance formed when a large number of six-membered rings composed of carbon atoms are linked together into a tubular shape. An ordinary carbon nanotube has a closed structure formed together with contained five-membered rings. Further, carbon nanotubes are divided into two classes including: a multi-walled carbon nanotube (MWCNT) in which a plurality of layers each formed by linked carbon atoms are stacked; and a single-walled carbon nanotube (SWCNT) farmed in the shape of a monolayer tube.
In the prior art, a method of preparing a film sample of a carbon nanotube-containing substance is known in which solution of carbon nanotubes solubilized using surfactant is prepared and then the prepared solution is dripped onto a plate and then dried up. Patent Document 1 discloses a technique of preparing a film sample of a carbon nanotube-containing substance in which single-walled carbon nanotubes are dispersed by using carboxymethylcellulose (CMC) as surfactant. The characteristics of carbon nanotube-containing substances can be investigated by using film samples prepared as described here.    [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-320828
In preparing a carbon nanotube-containing substance, it is difficult to disperse single-walled carbon nanotubes completely. Further, single-walled carbon nanotubes having been dispersed once can aggregate again so as to form aggregates. Thus, the degree of dispersion corresponding to a ratio of single-walled carbon nanotubes that do not form aggregates and are in an isolated state within single-walled carbon nanotubes contained in a carbon nanotube-containing substance having been prepared can have various values depending on the carbon nanotube-containing substance. Aggregates of single-walled carbon nanotubes do not generate photoluminescence. Thus, when a carbon nanotube-containing substance is to be used as a luminescence material, the degree of dispersion of single-walled carbon nanotubes affects the quality of the luminescence material. Accordingly, evaluation is necessary on the degree of dispersion of single-walled carbon nanotubes in a carbon nanotube-containing substance. The width of a peak or the like observed by photoluminescence spectrometry or absorption spectrophotometry of a carbon nanotube-containing substance corresponds to the degree of dispersion. Nevertheless, since the peak shape can vary also owing to factors other than the degree of dispersion, clear evaluation of the degree of dispersion has been difficult.